This disclosure relates to power supply clamps.
Power supply clamps are used to provide a switchable low impedance path between power rails of an electronic circuit, for example to provide at least some protection against damage due to electrostatic discharge (ESD) events.
There are many potential causes of ESD events. Two such potential causes which have rather different temporal characteristics are electrostatic discharges from a charged external item (such as the human body, or an external device or machine) touching the electronic circuit, and electrostatic discharges resulting from a charge retained by the electronic circuit itself being discharged to ground, for example by one or more terminals or pins of the circuit touching an earthed surface. Typically, ESD events relating to charged humans or external machines have a slower rise time and a slower decay (which is to say, they last longer) than ESD events relating to charged devices being discharged to ground.
Each individual ESD event has parameters (magnitude, temporal characteristics and the like) which depend on its particular circumstances. But as part of the design of protection arrangements intended to alleviate the effects of an ESD, various models of typical (or, at least, reproducible) ESD events may be used. Two such models are the Human Body Model (HBM), which models a slow rise time, slow decay ESD event, and the Charged Device Model (CDM), which models a fast rise time, fast decay ESD event. ESD protection arrangements are often designed so as to protect against one or both of these modelled ESD events, and test procedures are available according to the models which can be applied to an electronic circuit under test to find out whether it can withstand the modelled ESD event.
ESD protection is relevant to many types of electronic circuit, but has particular relevance to semiconductor devices such as those implemented as integrated circuits. One reason is that the small physical size of any individual component within the integrated circuit can limit the instantaneous power which that component can safely dissipate or the voltage stress which the component can safely sustain (during an ESD event) without damage.
An example ESD event can occur in the form of a discharge across the power rails of an electronic circuit. As mentioned above, a power supply clamp can provide a switchable low impedance path between the power rails so that the low impedance path can be enabled when an ESD event is detected. The low impedance path can handle a high instantaneous current. In some examples, a large field effect transistor is used. Therefore, an example power supply clamping process involves detecting the ESD event promptly, switching the low impedance path so that a current impulse caused by the ESD event is passed by the low impedance path, maintaining the low impedance path for the duration of the ESD event, and then switching off the low impedance path so as to return to normal operation of the electronic circuit.